An automatic transmission mounted on a vehicle is provided with a plurality of planetary gear sets (planetary gear mechanisms), and a plurality of friction engaging elements, such as a clutch and a brake. The automatic transmission is constructed so that the friction engaging elements are selectively engaged with each other by a hydraulic control to switch between power transmission paths via respective planetary gear sets, thereby achieving a plurality of forward gear ratios and normally one reverse gear ratio.
In recent years, a torque converter tends to be eliminated from the automatic transmission because the transmission is demanded, for example, to have more selectable gear ratios, and to reduce the weight. Without the torque converter, the vehicle can smoothly start traveling without an engine stall by carrying out a slip control of at least one friction engaging element which is engaged at a first gear.
If carrying out the slip control of the friction engaging element engaged at the first gear when the vehicle starts traveling, since the brake of which a hydraulic chamber does not rotate is better in controllability when engaged, than the clutch of which a hydraulic chamber rotates, the slip control of the brake may be carried out.
Among the brakes constructed in this way, it is known that a piston which causes a friction plate to be engaged is biased and moved in the engaging direction by a spring in order to improve the response when the vehicle starts traveling.
For example, JP2017-150533A discloses a brake in which a piston causing a plurality of friction plates to be engaged is biased by a first spring and a second spring in the engaging direction from a released position to a first position a given distance away from the released position, is biased only by the second spring from the first position to a second position where the plurality of friction plates become in a zero clearance state, and is then biased by the hydraulic pressure for engagement from the second position to an engaged position to cause the plurality of friction plates to be engaged.
FIG. 17 illustrates a cross-sectional view of such a brake 200 in the automatic transmission. The brake 200 includes a plurality of friction plates 203 disposed between an inner stationary member 201 coupled to a transmission case and an outer rotary member 202 coupled to a given rotary member, and a piston 206 fitted into a cylinder 205 which is formed by an outer cylindrical part 204a, a flange part 204b, and an inner cylindrical part 204c of a housing 204 which is a part of the transmission case.
The brake 200 also includes a hydraulic chamber 207 for engagement to which hydraulic fluid for engagement which biases the piston 206 in the engaging direction is supplied, and a hydraulic chamber 208 for release which is disposed at the opposite side of the hydraulic chamber 207 with the piston 206 therebetween, and to which hydraulic fluid for release which biases the piston 206 in the releasing direction is supplied.
In the hydraulic chamber 207 for engagement, a first spring 209 and a second spring 210 which bias the piston 206 in the engaging direction are disposed. The second spring 210 is disposed inside a groove portion 204d formed in the outer cylindrical part 204a of the housing 204, and the first spring 209 is disposed radially inward of the second spring 210.
When engaging the brake 200, if the hydraulic pressure for release is released from a state in which the hydraulic pressure for engagement is released from the hydraulic chamber 207 for engagement, and hydraulic pressure for release is supplied to the hydraulic chamber 208 for release to move the piston 206 to the released position where the first spring 209 and the second spring 210 are compressed, the piston 206 is biased by the first spring 209 and the second spring 210 to be moved to the first position which is the given distance away from the released position in the engaging direction.
When the piston 206 reaches the first position, the piston 206 is then biased only by the first spring 209 to be moved from the first position to the second position where the plurality of friction plates 203 become in the zero clearance state. After the piston 206 reaches the second position, when the hydraulic pressure for engagement is supplied, the piston 206 is then biased by the hydraulic pressure for engagement to be moved to the engaged position where the plurality of friction plates 203 are engaged.
On the other hand, when releasing the brake 200, if the hydraulic pressure for engagement is released and the hydraulic pressure for release is supplied, from a state in which the hydraulic pressure for release is released from the hydraulic chamber 208 for release, and the hydraulic pressure for engagement is supplied to the hydraulic chamber 207 for engagement to move the piston 206 to the engaged position, the piston 206 is biased in the releasing direction and the piston 206 is moved to the released position where the first spring 209 and the second spring 210 are compressed.
In the brake 200, since the biasing force of the second spring 210 is set larger than the biasing force of the first spring 209, the piston 206 can be moved with sufficient response by the first spring 209 and the second spring 210 from the released position to the first position, and can then be moved with sufficient accuracy by the first spring 209 from the first position to the second position.
However, the automatic transmission provided with the brake disclosed in JP2017-150533A has a structure with a large axial dimension in that a hydraulic chamber for engagement to which hydraulic fluid for engagement which biases the piston in the engaging direction is supplied is disposed in the axial directions serially at the opposite side from the friction plates of the piston which causes the plurality of friction plates to be engaged disposed between the inner stationary member coupled to the transmission case and the outer rotary member.
In such an automatic transmission provided with the brake having the hydraulic chamber for engagement to which the hydraulic fluid is supplied, which is for engagement of the piston which causes to be engaged the plurality of friction plates disposed between the inner stationary member and the outer rotary member, by disposing the hydraulic chamber for engagement radially inward of the inner stationary member, downsizing the automatic transmission in its axial dimension is desirable.
In this case, it can be considered that an oil channel forming member forming a supply oil channel for engagement which supplies hydraulic fluid for engagement to the hydraulic chamber for engagement is provided radially inward of the hydraulic chamber for engagement disposed radially inward of the inner stationary member so as to be coupled to the transmission case. Thus, the hydraulic fluid for engagement is supplied from the transmission case through the supply oil channel for engagement of the oil channel forming member to the hydraulic chamber for engagement.
However, when the oil channel forming member is formed radially inward of the hydraulic chamber for engagement, since the radial dimension increases due to the oil channel forming member, it is desired to avoid the increase in the radial dimension due to the oil channel forming member and downsize the radial dimension.